1. Field of the Invention
The present invention relates to a tire pressure detecting system for detecting a tire pressure and enabling a driver to recognize the tire pressure even when a vehicle is moving.
2. Description of Related Art
As one of tire pressure detecting systems that enable the driver to recognize the tire pressure even when the vehicle is moving, for example, a “TIRE PRESSURE ALARM SYSTEM” disclosed in JP-A-10-44726 is known.
The above-described technique proposes the tire pressure alarm system for measuring tire pressures including: a valve stem for inflating a tire; a case being disposed at the base of the valve stem; and a transmission unit having a pressure detecting unit, a signal processing circuit and a battery and being stored in a case, wherein the driver is notified when an abnormality is found in the tire pressure or when the tire pressure has fallen to a value below a predetermined value.
However, with the tire pressure alarm system described above, notification is only made, for example, when the tire pressure has fallen to the value below the predetermined value.
The tire pressure may fall gradually, abruptly, and at a speed in-between, depending on the cases.
For example, when a minute hole is formed on a tire, and thus air leakage occurs, the tire pressure falls gradually. However, it may not be sufficient if it is determined to be normal because the tire pressure is not less than a threshold value of the pressure.
In the tire pressure alarm system described above, in many cases, a time interval until alarm or the like is issued for notifying the driver that the tire pressure has fallen to a value below the predetermined value is set to a long time interval in order to reduce consumption of the battery.
However, if the time interval until the alarm or the like issued is kept long even for the cases when the air pressure in the tire has abruptly fallen and the rate of reduction of the tire pressure per unit time is outstandingly high, it is difficult to notify the user of an abnormality when the tire pressure has fallen in a short time. There may be a case in which the tire pressure becomes zero before the alarm is issued. In other words, a technique to notify the driver quickly when the pressure has abruptly fallen to a value below the threshold pressure is desired.
In addition, when the pressure detecting unit is a sensor employing a diaphragm and the like, it is exposed on one side to the ambient air, and on the other side to the interior of the tire. Therefore, the tire pressure indicated by the sensor may be an inappropriate value due to the ambient air pressure, which will be described in conjunction with figures shown below.
FIG. 16a is explanatory drawings illustrating an example of an air pressure sensor used for the tire pressure alarm system of the related art. Although the air pressure sensor shown here is a manometer for convenience, it may be a tire pressure gauge that is commercially available.
An air pressure sensor 401 is the sensor exposed on one side to the ambient air and on the other side to the interior of a tire 402. The air pressure of the tire 402 is set to 200 kPa in gauge pressure at high altitudes of 80 kPa for ambient air pressure. In this case, the air pressure is 280 kPa in an absolute pressure.
FIG. 16b is explanatory drawings illustrating an example of an air pressure sensor used for the tire pressure alarm system of the related art. Although the air pressure sensor shown here is a manometer for convenience, it may be a tire pressure gauge that is commercially available.
The tire 402 set to 200 kPa in air pressure at high altitudes of 80 kPa for ambient pressure is moved to a level ground of 100 kPa in ambient pressure as shown by an outline arrow. The air pressure in the tire 402 is stable at 280 kPa in absolute pressure, but becomes 180 kPa in gauge pressure.
In other words, when a vehicle wherein the tire pressure (gauge pressure) is set at a high altitude under low ambient air pressure is moved to the level ground under the ambient air pressure higher than the pressure at high altitudes, the value of the indicated tire pressure (gauge pressure) is lower by an amount corresponding to an increase in the ambient air pressure. In contrast thereto, when the vehicle wherein the tire pressure (gauge pressure) is set at the level ground under a normal atmospheric pressure is moved to high altitudes under low ambient pressure, the value of the indicated tire pressure (gauge pressure) is higher by an amount corresponding to a decrease in the ambient air pressure.
Therefore, it is desired that the tire pressure alarm system issues a warning when the value of the tire pressure (gauge pressure) has fallen to a value below a value predetermined under certain conditions both at high altitudes and at level ground.